Thread: Sawstop

Originally Posted by rrich

It was either June or July 2005 that I assisted installing 8 or 10 SawStop table saws at a Community College.
(A two year institution, years 13 and 14 if you will.)...

In USA they are known as Two Year Colleges or Community Colleges.

In Australia they are known as TAFEs - Technical And Further Education colleges.

Essentially the same thing.

Originally Posted by woodPixel

Interesting that it doesn't use a disk brake like arrangement with a loaded heavy spring to drop/pull the armature down/away.

It would save the blade and cost to re-arm would be zero.


edit - An electro-magnetic brake would do the same job. The ones metal workers use are simply ferocious (haha! that pun is awful!).

I think its a mix of space, size, ease of changing blades, cost and unless the brake is over sized... getting it to clamp and stop instantly (millie seconds) its a big ask and the clamping pressures huge.

Originally Posted by havabeer69

I think its a mix of space, size, ease of changing blades, cost and unless the brake is over sized... getting it to clamp and stop instantly (millie seconds) its a big ask and the clamping pressures huge.

Not to mention the very high potential for resin, dust and debris to inhibit the effective braking.

Originally Posted by rrich

Further into the conversation, the instructors belief was that the blade was touched by a metal measuring tape. He also felt that the method students were taught to change blades may be a problem. (Blade on the arbor, finger tight the nut, wrench on the nut and braced on throat opening, pinch blade between thumb and index finger pulling the blade toward the operator) He felt that if the blade was pulled too fast it would trip the cartridge and drop the blade. DUNNO But the schools safety record doesn't equate to that many blade drops.

This does not seem at all plausible to me .... if the blade is stationary then rotated at very low rpm (compared to running 5 to 7000 rpm) and the saw is isolated as it should be when changing a blade how can this possibly occur????

Originally Posted by Mobyturns

Not to mention the very high potential for resin, dust and debris to inhibit the effective braking.

Can't be be any worse than a motor vehicles disc brake exposed to road dust/grit mud etc.

One problem with fitting a disc brake to a table saw spindle is that this would inhibits the depth of the cut as the disc has to be kept under the table otherwise it will have to poke above the table and interfere with the work. This is not a problem in the case of a lathe or other machinery which can have a dis mounted at the other end of the spindle without interfering with the work.

The disc could be mounted off axis eg on the motor, but then the system is using the belts to stop the blade which would not be as instantaneous as a caliper direct on a blade.

FWIW I have a home made foot operated disc brake on my bandsaw motor. The brake pads are from a Colt Galant, the caliber is from hospital gurney and the rest is home made.
With the 6tpi 25mm Blade on the saw I can comfortably stop the blade in about 3s - I could stop it faster (eg <2s) but I worry about the stresses involved. For a BS it would be better to have the disc on one of the wheels.
Painted-Brake.jpg

No more so than the rotations from a little disk brake on a mountain bike.

I'm a Chunky Lad and love biking. The disk brakes when they first came out looked too tiny to be effective, but they pull up my mighty frame and bike quick smart, even down hill at great velocities (arguably the very best time for such up-pulling).

A 12" saw blade at 6000 rpm is NOTHING compared to the masses I just described

An electric disk brake would clamp that saw blade to a dead stop absolutely instantaneously.


On an aside, I've been seeing more electric disk brakes everywhere recently - trailers, caravans, those little tents-in-a-trailer, cars (drive by wire!) and in the Formula 1 they are absolute.

Seems to me that a retrofit kit for ordinary tablesaws should be dead easy from an engineering perspective.

Here are some interesting BOTE calcs

Kinetic energy of 120kg person+bike at 60kph = 15360 joules.
To stop that bike rider combo in 1s requires a power dissipation rate of 15.36kW

According to several web sources in practice the best a pro rider can stop in and still stay on a bike is between 1.5 and 2 secs so the power dissipation required is about half that above.
Mind you that will NOT be for 120kg bike+rider - ie more like a 70kg cyclist and 10kg bike so lets half that again.

The KE of 300mm saw blade + blade Pulley + motor pulley + motor spindle all travelling at 6000 rpm is only about 100J

BUT

If you want to stop the saw blade + blade pulley + motor pulley + motor spindle, in 5ms requires the ability to dissipate 20kW over that time!

Not saying its not possible (i'd even be surprised if if it can't be done) but its not as straight forward as it may appear.

Originally Posted by BobL

Can't be be any worse than a motor vehicles disc brake exposed to road dust/grit mud etc.

Ah, it is very different!

A motor vehicles disk brakes are designed to slow / stop a vehicle in an acceptable period of many seconds and is not designed to prevent all / any injury. There are other safety systems in a modern vehicle that provide a "safety system" - seat belts, air bags, side intrusion bars etc.

The SawStop scenario is to "instantly" stop the saw blade, and retract it into the table to prevent any injury. It is the only safety mechanism in place, so must deploy with 100% reliability to be effective. If a "disk brake" mechanism is to perform the same functions in the same millisecond time span then it would want to be a pretty well designed unit like nothing currently available.

Originally Posted by Mobyturns

A motor vehicles disk brakes are designed to slow / stop a vehicle in an acceptable period of many seconds and is not designed to prevent all / any injury. There are other safety systems in a modern vehicle that provide a "safety system" - seat belts, air bags, side intrusion bars etc.

Not quite. A motor vehicles disc brake system is designed to slow/stop etc. . . . . .
The Disc brakes themselves are designed to stop the wheels ASAP - it's the auxiliary components like ABS that releases brake pressure and just keeps the wheels turning to reduce skidding and maintain steerage/control etc

A modern motor vehicle disc brake can instantly lock a rotating vehicle wheel if no ABS is present and there's no forward vehicle momentum OR vehicle weight to generate wheel/road friction that keeps the wheel turning. Just think about a car with no ABS driving on ice. A stomp on the disc brake will easily lock the wheels.

Now replace the pedal with a 1ms relay to cut the motor mains and activate an EM actuator at the calliper. BANG ! It's not going to be kind to wheel bearings and shafts etc if this is the only way to stop the saw but the same applies to the saw stop. These devices are meant for emergencies not a regular way to stop rotation.

BUT going back to what I was referring to in terms of the dirt and dust on the rotor and pads etc this makes very little difference - just look at rally cars etc on those dusty rods etc.

FWIW a 750kg F1 car at 200 kph can stop in about 3s during which each wheel must generate of ~100 kW of braking power - their brakes can deliver away more stopping power than this but just like an regular vehicle they are no good if they lock up - which is what we want to do for a TS.

20kW for a saw blade over 5ms starting to look doable but I suspect more expensive than a bit of honeycomb Al and an explosive bolt.

I was also thinking about the rotation of the blade in all of this.

A 12" (300mm) blade at 6000 rpm is moving at ~ 95 m/s .... a fair clip! (30pi=94.3cm x 100)

If a saw takes one millisecond to detect, fire and engage to an utter stop, that blade has moved 9.5cm

That's a lot of chomping on a finger. 10cm of blade has a few teeth in it.

Hence the pulling down/away mechanism.

It all makes sense now why its designed the way it is. Stopping it alone is not enough.

Originally Posted by Mobyturns

"An oscillator generates a 12-volt, 200-kilohertz (kHz) pulsed electrical signal, which is applied to a small plate on one side of the blade. The signal is transferred to the blade by capacitive coupling. A plate on the other side of the blade picks up the signal and sends it to a threshold detector. If a human contacts the blade, the signal will fall below the threshold. After signal loss for 25 micro seconds (µs), the detector will fire. A tooth on a 10-inch circular blade rotating at 4000 RPM will stay in contact with the approximate width of a fingertip for 100 µs. The 200-kHz signal will have up to 10 pulses during that time, and should be able to detect contact with just one tooth.[5] When the brake activates, a spring pushes an aluminum block into the blade. The block is normally held away from the blade by a wire, but during braking an electric current instantly melts the wire, similar to a fuse blowing."

SawStop - Wikipedia

the bit I've highlighted in bold italic is as I understand it is the basis of Steve Glass's patent.
"anyone" can drop and stop a saw blade in 100 millionths of a second -- the energy involved is not that great.
the key technology that Glass patented is the 200 kHz pulsing detector. As I understand the patent dispute with the Bosch Reaxx saw, the Reaxx used the same or sufficiently similar technology to breach SawStop's patents.

Originally Posted by woodPixel

I was also thinking about the rotation of the blade in all of this.

A 12" (300mm) blade at 6000 rpm is moving at ~ 95 m/s .... a fair clip! (30pi=94.3cm x 100)

If a saw takes one millisecond to detect, fire and engage to an utter stop, that blade has moved 9.5cm

That's a lot of chomping on a finger. 10cm of blade has a few teeth in it.

Hence the pulling down/away mechanism.

It all makes sense now why its designed the way it is. Stopping it alone is not enough.

dropping the blade is sufficient. There is really no need to stop the blade.
Afterall, we are talking about a system that works within 50 MICRO (millionths) seconds, not 1 millisecond

I read both of the Steve Gass patents.
IIRC, the basis of both patents is sensing flesh coming into contact with the blade removes the blade from contact with flesh.
When I read that, I realized that the patent must expire before there will be competition.

However, the system mentioned here is flesh near the blade activates the mechanism. So maybe this system will be practical. As far as sitting on a jury for patent infringement and the word "contact" could be the deciding factor. I know that if the Gass system requires blade contact and this system does not, IMNSHO there is no infringement. But Gass is a lawyer with resources. A lengthy court battle could easily bankrupt a small start up company.

Originally Posted by RossM

Yes - had exactly this issue with bone dry radiata 50 x 100. No metal, no knots, no sap, no moisture, no fingers, no contamination any where. SawStop blade was fitted.

I sent the cartridge & the off-cut to Cabitec with a complaint, but they claim that the data analysis of the chip in the cartridge showed it had hit metal. However the offcut clearly showed that there was no metal in the timber. Was VERY unimpressed, as they refused to replace the cartridge or blade under warranty. (I am tempted to get a saw doctor to replace the carbide tooth that was removed during braking.) So @haveabeer69 is wrong, they won't replace the cartridge unless their data collection showed it has hit flesh.

When I finally get a big shed I'll be ditching the SawStop JobSite saw & trading up to a Euro Slider.

I got the same answer, they assured me that it fired due to contacting metal.... I know i can't change anyones mind and it is what it is, bt there was no metal anywhere near the blade and the the timber was clean..... I wanted to find what I did wrong to ensure I do not repeat the issue. I know for sure I did not make contact with metal.....so now I am second guessing myself every time I run a bit of timber through just waiting for the next bang! Shame, the saw otherwise is brilliant.